Introduction
F-18 fluoro-2-deoxyglucose positron emission tomography/computed tomography (
18F-FDG PET/CT) has become an indispensable method for the diagnosis, staging, and response evaluation of many malignancies [
1‐
3]. In gastric cancer (GC),
18F-FDG PET/CT is a useful tool for the diagnosis of recurrent disease after curative surgery [
4‐
7]. However, the role of preoperative
18F-FDG PET/CT is not yet fully established. While the National Comprehensive Cancer Network guidelines recommend the use of preoperative
18F-FDG PET/CT in GC patients to rule out distant metastasis, the prognostic impact of preoperative
18F-FDG PET/CT remains a matter of debate. Several reports suggest a potential prognostic role for preoperative
18F-FDG PET/CT, while others argue against this [
8‐
10]. These discrepancies may be due in part to small sample sizes and heterogeneous patient populations in different studies. Notably,
18F-FDG PET/CT has low sensitivity in detecting early GC (EGC) and signet ring cell (SRC) GC, but many previous studies overlooked this, including heterogeneous populations in the studies and analyzing the clinical characteristics of the population as a whole. Because SRC GC is a unique histologic subtype of GC with a distinct tumor biology and bioenergetics, it should be analyzed separately [
11‐
14].
The present study evaluated the prognostic impact of SUVmax in preoperative 18F-FDG PET/CT of advanced GC (AGC) according to histologic subtype, with a focus on the differences between tubular adenocarcinoma and SRC carcinoma.
Materials and methods
Patient selection
Between January 2006 and December 2013, patients with GC who underwent 18F-FDG-PET/CT and subsequent curative surgical resection at Yonsei Cancer Center, Severance Hospital, Seoul, Korea, were enrolled in the study. A predesigned data collection format was utilized to extract data from a prospectively maintained database. The main eligibility criteria were as follows: (1) pathologically confirmed AGC of tubular adenocarcinoma or SRC-histologic subtype; (2) available documented information regarding the primary tumor site, postoperative pathological stage, surgery, recurrence, and survival; and (3) patients who received curative resection including those who presented with enlarged paraaortic lymph nodes having radical surgical resection with a curative aim accompanied by paraaortic lymph node dissection. The main exclusion criteria were as follows: (1) patients with EGC; (2) patients who received neoadjuvant chemo- or radio-therapy; and (3) patients with multiple primary cancers. After applying these criteria, 727 of the original 1605 patients were included in the final analysis (Fig. S1). The pathological stage was classified according to the American Joint Committee on Cancer (AJCC) staging manual (7th edition). This study was approved by the Institutional Review Board of Severance Hospital (#2015-1751-001). For a validation cohort, AGC patients who underwent preoperative 18F-FDG-PET/CT and curative surgical resection at CHA Bundang Medical Center, Seongnam, Korea, between March 2007 and February 2014, were enrolled in the study. Data acquisition and analysis were adopted identically as in the aforementioned institution.
The WHO and the Lauren classifications were used for the histopathological evaluation of surgical specimens. Tubular adenocarcinoma was additionally classified as being well, moderately, or poorly differentiated according to the WHO classification. Accordingly, we divided the patients into three groups for further analyses: well or moderately differentiated (WMD), poorly differentiated (PD), and SRC. In terms of the Lauren classification, the tumors were classified as intestinal, diffuse, or mixed type.
18F-FDG PET/CT and image analyses
All 18F-FDG PET/CT were performed with either the Discovery STe PET/CT (GE Healthcare, Milwaukee, WI, USA) or the Biograph TruePoint 40 PET/CT (Siemens Healthcare, Erlangen, Germany). All patients fasted for at least 6 h before the scan, and the glucose level in the peripheral blood of all patients was confirmed to be 140 mg/dL or less before 18F-FDG injection. Approximately, 5.5 MBq 18F-FDG/kg body weight was administered intravenously 1 hour before image acquisition. After the initial low-dose computed tomography (CT) (Discovery STe: 30 mA, 140 kVp, Biograph TruePoint: 36 mA, 120 kVp), standard PET/CT imaging was performed from the neck to the proximal thighs with acquisition times of 2.5 min/bed position for the Biograph Truepoint 40 PET/CT and 3 min/bed position for the Discovery STe scanner in three-dimensional mode. Images were then reconstructed using ordered subset expectation maximization (2 iterations, 20 subsets).
The images were retrospectively reviewed on a GE AW 4.0 workstation by two experienced nuclear medicine specialists (A.C. and M.Y.) who were unaware of the patients’ clinical information, except for the diagnosis of GC. The evaluation of 18F-FDG PET/CT images was performed in two steps. First, 18F-FDG PET/CT images of all patients were visually assessed and the patients were classified as positive or negative with respect to 18F-FDG uptake in the primary tumor. Lesions showing focally increased 18F-FDG uptake that exceeded the uptake in the surrounding stomach wall and corresponding cancer lesions as observed by contrast-enhanced CT images and gastroduodenoscopy were classified as positive 18F-FDG uptake. Focally or diffusely increased 18F-FDG that was indistinguishable from physiological gastric wall uptake was judged to be negative 18F-FDG uptake. After the visual assessment, the maximum standardized uptake value (SUVmax) of the primary lesion was obtained and recorded for semi-quantitative analysis.
For the validation cohort, the PET/CT imaging for 173 AGC patients from CHA Bundang Medical Center was performed with Biograph mCT 128 scanner (Siemens Medical Solutions, Knoxville, TN, USA). Initial low-dose CT scans for attenuation correction (120 kV, 120 mA, 3 mm section width, 3 mm collimation) and PET/CT scans of same area with three-dimensional mode were acquired consecutively. Images were reconstructed on 400 × 400 matrices using the TrueX algorithm plus time-of-flight (TOF) reconstruction and analyzed using a dedicated workstation and software (Syngo.via, Siemens Medical Solutions, Knoxville, TN, USA). Unless otherwise stated, all other methods applied for image acquisition and data analysis were adopted identically as in the aforementioned institution.
Statistical analysis
The cut-off date was December 31, 2015. The mean SUVmax was compared according to the patients’ basic demographic and clinical characteristics using independent sample t tests or analysis of variance. For pairwise comparisons of each level of categorical variables, the statistical significance was adjusted for inflated type I errors from multiple comparisons using the Bonferroni method.
Relapse-free survival (RFS) was measured from the time of surgery to initial tumor relapse (either local or distant) or death from any cause, and overall survival (OS) was calculated as the time from surgery to death from any cause or to the last follow-up date. Survival outcomes of the group with high SUV
max were compared with survival outcomes of the group with low SUV
max based on the median SUV
max for each histologic subgroup using the log-rank test. The Cox proportional hazards model was used for multivariable analysis of prognostic factors, including age at diagnosis, sex, stage, and PET/CT SUV
max. To determine additional associations between SUV
max and survival outcomes, we examined the Cox regression model using the penalized spline smoothing method as described previously [
15]. The performance of prognostic models was measured by Harrell’s c-index. We assessed model calibration by plotting the model-predicted- and actual observed 3- and 5-year RFS probabilities as calculated using the Kaplan–Meier method. The bootstrapping method with 1000 re-samples was used for adjusting bias and checking the interval validation. Statistical significance was set as
P < 0.05 for all analyses. All statistical analyses were performed using SPSS version 20.0 (SPSS, Chicago, IL, USA), SAS version 9.4 (SAS Institute Inc., Cary, NC, USA), and R package, version 3.2.4 (
http://www.R-project.org).
Discussion
Gastric cancer is increasingly recognized as a heterogeneous disease [
11,
14,
16,
17]. Classically, it is classified according to its histology, i.e., as intestinal type or diffuse type [
18]. Intestinal-type GC is more predominant in older people and in men, whereas diffuse-type GC is more frequently found in younger women. Recently, genomic data is widely utilized to develop molecular classification systems for GC. The TCGA Research Network proposed a classification system to distinguish GC into four subtypes: (1) Epstein–Barr virus (EBV)-positive, with the highest DNA methylation levels; (2) microsatellite instability (MSI), characterized by hypermutated tumors; (3) genomic stability (GS), which represents 20% of GC and comprises the majority of diffuse-type GC, has the most abundant CDH1 mutations and also shows increased RHOA mutations and CLDN18–ARHGAP fusions; and (4) chromosomal instability (CIN), which accounts for 50% of patients and is characterized by frequent TP53 mutations and high percentage of intestinal-type GC [
16,
19].
However, current clinical practice does not take this heterogeneity into account; rather, GC is regarded as a single type of malignancy, and a one-size-fits-all approach is applied. Although some previous studies have shown that the SUV
max in PET/CT can differ markedly according to the histologic subtype, most studies still evaluate the prognostic impact of SUV
max by considering GC to be a single disease entity rather than categorizing it into the various histologic subtypes. Not surprisingly, this has resulted in inconsistencies between studies. Furthermore, despite the large number of studies that have already observed poor
18F-FDG PET uptake in patients with EGC only having mucosal or submucosal invasion, most studies still enroll patients with EGC. The largest preoperative study to date was reported by Lee et al., who evaluated the prognostic impact of PET/CT in 271 GC patients [
8]. Because approximately half of the enrolled patients had EGC, 45% of the patients had no detectable
18F-FDG uptake, and so only the remaining 149 patients were available for further analysis. Consequently, the subgroup analysis was limited by the small sample size.
To overcome the limitations of previous studies, the present study prospectively collected data from more than 700 patients with AGC while excluding patients with EGC. Moreover, to avoid oversimplification, the patients were divided and analyzed according to their histologic subtypes. Furthermore, the prognostic impact of SUVmax was evaluated not only as a dichotomous variable as determined by the median SUVmax but also as a continuous variable by analyzing p-spline curves. As a result, we were able to reveal the distinct prognostic impact of SUVmax in 18F-FDG PET/CT according to histologic subtype.
First, diffuse-type AGC with SRC histology had lower SUVmax compared with intestinal-type AGC with WMD histology, which is in agreement with previous studies. Second, although the SUVmax of diffuse-type AGC was lower than that of intestinal type, it had a significant prognostic impact in terms of survival outcome. On the other hand, the SUVmax of intestinal-type AGC was more directly correlated with primary tumor size than diffuse-type AGC, but it did not have any prognostic impact. These finding provide a clue to explaining previous discrepancies regarding the prognostic impact of 18F-FDG PET/CT in GC patients. Finally, we established and validated a novel model that utilizes the preoperative SUVmax to predict tumor recurrence after surgery in patients with SRC or diffuse type, which may be a useful tool for clinical application.
Each histologic subtype of GC differs in its biology, especially in its metabolic profiles, which leads to different
18F-FDG uptake patterns [
20]. Among the various histologic types of GC, SRC stands out as a unique subtype due to its distinct molecular and metabolic features. In terms of the glucose transporter GLUT-1, SRC is reported to express GLUT-1 at lower levels than WMD adenocarcinoma, leading to reduced
18F-FDG uptake [
21,
22]. In addition, SRC has lower levels of the pyruvate kinase M2 isoform (PKM2) compared with other histologic subtypes; PKM2 is responsible for ATP production in the last step of glycolysis [
12]. Furthermore, PKM2 expression is correlated with poor prognosis in SRC, while other subtypes are not. These metabolic characteristics help explain the different patterns and prognostic values of
18F-FDG PET/CT in different histologic subtypes of GC, and our data highlight the importance of dividing GC into different histologic subtypes before PET/CT analysis.
The main limitation of our study is the retrospective nature of data collection. Although we verified our findings in two independent cancer centers in Korea, more studies are needed to validate our findings in a prospective cohort. Especially, this study did not include patients who underwent preoperative treatment. Therefore, it is necessary to evaluate the role of PET/CT in Western patients who received preoperative treatment with other studies. In addition, this study only included patients with AGC, thus most patients received adjuvant chemotherapy. There are limitations in analysis of the effect of adjuvant chemotherapy on the prognostic remodeling. Second, volumetric PET parameters were not measured due to the large number of cases. Additional studies are needed to further evaluate the value of volumetric PET parameters rather than SUVmax for predicting clinical outcomes in intestinal-type AGC.
In conclusion, this study demonstrated the differential patterns and prognostic impact of preoperative PET/CT SUVmax in AGC according to histologic type. Although the SUVmax did not have significant prognostic impact in WMD- and intestinal-type AGC, higher preoperative SUVmax indicated poorer prognosis in SRC and diffuse-type AGC. Novel predictive models for recurrence probability can be provided based on the preoperative SUVmax in patients with SRC or diffuse-type AGC. To validate these findings, we are preparing a prospective trial. If the results of this study are confirmed in a prospective trial, SRC or diffuse-type gastric cancer patients with high SUVmax should be stratified in adjuvant chemotherapy. Ultimately, a clinical trial in which novel or more intensive therapy approaches are applied to SRC and diffuse-type AGC patients with high SUVmax should be performed.